The present invention is described in connection with the preferred embodiment, as a drive mechanism for a transducer head of a magnetic disk memory. It is to be understood, however, that the invention is, in certain instances, applicable to any motor drive system.
Information is written into and read from circular concentric tracks of magnetic disk memories by read/write transducers, located an extremely small distance above faces of disks of the memory. The transducers are driven radially relative to a turning axis of the disk faces to enable data to be written into or read from all of the concentric tracks on each of the disk faces. During read and write operations, the disks are driven at constant rotational velocity.
A single read/write transducer is associated with each face of a magnetic disk in such a memory. The transducer is mounted on a platform which travels parallel to the disk face and is driven by a linear or rotary electric motor. The platform includes two principal components, namely a head including the transducers and a suspension device. One extremity of the suspension device is an integral part of the head, while the other extremity of the suspension device is an integral part of a rigid arm, in turn an integral part of an output shaft of an electric motor.
Generally, the head including the read/write transducer or transducers associated with one of the disk faces is shaped as a relatively flat rectangular parallelipiped, having an underside facing the disk and containing the transducer. The head includes a large, upper side parallel to the underside, and containing the ends of electric wires connected to the transducer and to electronic read and/or write circuits of the disk memory. A platform of this general type is described in U.S. Pat. No. 4,261,024, commonly assigned with the present invention.
When data are written into and read from the disk, the transducer hovers above the disk face with which it is associated, whereby a layer of compressed air is formed between the underside of the platform head and the disk face. The compressed air layer prevents the head from touching the disk, thereby preventing disk damage. The distance or hovering height between the disk face and the underside of the head is referred to as h. When the platform is at the hovering position, a dynamic balance is provided by opposing the force created by the air cushion on the underside of the platform with a force, referred to as a "load force" directed in the opposite direction from the air cushion force. The load force is applied to the top of the head and has a modulus equal to that of the force established by the air cushion. The load force is relatively light, on the order of 10 to 15 grams, and is supplied by a load plate, an integral part of a fixed, rigid arm for carrying the head. It is difficult to maintain the head at the hovering position. When the disk memory is not operating, the head is parked or located at an "idle position" near the disk periphery off of the disk. In the idle position, the separation height H between the underside of the head and the plane of the disk associated with the head is considerably greater than the hovering height h. In order for the head to begin to hover, the head must be moved from the idle position, at a height H several tenths of a millimeter above the disk face, to a stable hovering position at a height h, several tenths of a micron above the disk surface. Lowering the head from the idle position to the hovering position is difficult, primarily because of air turbulence near the face of the disk.
There is an increasing tendency to employ magnetic disk memory systems having rotary, rather than linear, activated arms for displacing transducer containing heads and for loading the heads into the hovering position. The rotary arm is typically an integral part of a drive mechanism coupled to a rotary electric motor. The drive mechanism typically includes a structure for lifting and lowering the head between the hovering and idle positions. Such a system is described, for example, in copending, commonly assigned U.S. application Ser. No. 467,202, filed, Feb. 16, 1983, U.S. Pat. No. 4,571,648.
In the system described in the copending U.S. patent application, the positioning mechanism is mounted on a ball bearings. Each head is loaded into the hovering position by a load plate having one extremity connected by an articulated joint to an arm, forming an integral part of a rotary positioning mechanism. A head suspension device is mounted at the other extremity of the load plate. The load plate is mechanically connected to a wire on which rests a follower mounted on a extremity of a spring plate connected by an articulated joint to the arm which is an integral part of the drive mechanism. The follower engages a camming surface or profile of a fixed cam. In one preferred embodiment, the camming surface includes two break points, to divide the camming surface into three distinct sections.
A head is radially displaced and loaded into the hovering position from the idle position in three sequential phases. Each of the three sequential phases corresponds to a position of the cam follower on one of the three sections of cammed surface. The three phases defined by the position of the follower and the three sections of the cam are:
1. The unlocking phase, wherein the rotary drive mechanism is activated between an immobilized or locked condition and a released condition, at an idle height or distance H above the plane of the disk face associated with the head and at a radial position beyond the disk periphery.
2. Driving the head radially, between the position beyond the disk periphery to a position in proximity to the periphery of the disk, while maintaining the head at the idle height H above the plane of the disk.
3. Changing the height of the head between the idle height H and the hovering height h while driving the head radially from in proximity to the disk periphery to a region above the edge of where concentric data tracks are provided in the disk.
In phases 1, 2, and 3, the cam follower is driven across three distinct and different portions of the camming surface. In the first and third phases, the rotary drive mechanism drives the cam follower through less than a complete revolution, while the follower is driven through one complete revolution in phase 2, which defines the center section of the camming surface or profile. In phase 3, a tensile load exerted by the follower on a load plate coupled thereto and carrying the head gradually decreases as the head moves from the idle to the hovering position, to enable the plate and head to be lowered gradually toward the face of the disk. The load plate pulls the head toward the face of the disk in such a way that when the follower reaches the end of its travel, at the end of the third section of the camming surface, the head is in a stable hovering position. When the head reaches the stable hovering position above the face of the disk, the head is said to be loaded. When the head is loaded, the follower no longer engages the camming surface.
When the head is retracted from the loaded or hovering position to the locked position, the cam follower traverses the three sections of the cam surface in the opposite direction, whereby the sequence goes from phase 3 to phase 2, to phase 1. This is referred to as the unloading phase, in contrast to the sequence from phase 1 to phase 2 to phase 3, which is termed the loading phase.
The head displacement and load system may be likened to a movable system which travels along a given trajectory formed by three sections of a cam surface. The movable system including the head displacement and load system is subsequently referred to herein as a mobile component formed of a follower traveling along a cam profile or cam surface and having the same mass and inertia as a head displacement and loading system.
It has been found that as the mobile component completes one cycle over a trajectory of the cam surface and begins another cycle, the resistive, frictional forces opposing the motion of the mobile component along the cam surface are subject to variations up to 50%. Thus, the frictional forces may vary 10 to 15 grams between adjacent cycles of the same machine. Similar observations have been made for different disk memories. The variations are primarily due to changes in environmental factors, such as temperature, humidity, and amount of static electricity in the air.
As the mobile component travels along the cam surface, certain major problems have been found to subsist because of the considerable variation in the frictional forces opposing the motion of the follower along the cam surface. While the mobile component is being unlocked, i.e., while the follower travels along the first section of the cam surface, it has been found desirable to determine when the follower reaches the intersection between the first and second cam sections. Because of the possible variations in frictional forces between adjacent locking and unloading cycles, the cam follower is likely to reach the first break point at different times, even though the same voltage and current are applied to the electric motor driving the mechanism including the follower.
When the follower travels along the second section of the cam surface and reaches the second break point, the velocity of the mobile component should fall within two predetermined values. If the velocity of the mobile component is less than a predetermined value, there is a likelihood that the cam follower will travel backward away from the second break point. If, however, the velocity of the mobile component when the cam follower reaches the second break point is greater than a predetermined value, it is likely that the motor speed is excessive, causing the head to crash onto the disk as it is loaded during the third phase. This obviously has devastating results to both the disk and the head. While the head is being loaded onto the disk, from the idle position, H to the hovering height h during the third phase, the head velocity must fall in a range between a pair of predetermined values (e.g. between 8 and 16 millimeters per second). If the head velocity is excessive during the third phase, there is a risk that the head will crash onto the disk. If the head falls at too slow a rate during the third phase, the head and the mechanism carrying it are likely to oscillate unstably, which may also cause the head to strike the disk.
Thus, it is important for the motor shaft to be displaced at a controlled velocity while the follower is moved on the third section of the cam surface. The motor shaft is likely to be subjected to a sudden drive force as soon as the cam follower has cleared the second break point of the cam surface, as it reaches the third phase. This is because there is a tendency to impart a sudden acceleration to the cam follower when it makes the transition from the second to the third section of the cam surface. It can be shown that the driving power to the motor decreases as a function of the distance of the mobile component relative to the second break point, i.e., the drive power decreases as the distance of the cam follower from the second break point increases as the cam follower moves along the third section of the camming surface.